Decouple registration

NewServeMux above declares an explicit dependency on EchoHandler. This is an unnecessarily tight coupling. Does the ServeMux really need to know the exact handler implementation? If we want to write tests for ServeMux, we shouldn't have to construct an EchoHandler.

Let's try to fix this.

1. Define a Route type in your main.go. This is an extension of http.Handler where the handler knows its registration path.

   // Route is an http.Handler that knows the mux pattern
   // under which it will be registered.
   type Route interface {
     http.Handler
   
     // Pattern reports the path at which this is registered.
     Pattern() string
   }
   

2. Modify EchoHandler to implement this interface.

   func (*EchoHandler) Pattern() string {
     return "/echo"
   }
   

3. In main(), annotate the NewEchoHandler entry to state that the handler should be provided as a Route.

       fx.Provide(
         NewHTTPServer,
         NewServeMux,
         fx.Annotate(
           NewEchoHandler,
           fx.As(new(Route)),
         ),
         zap.NewExample,
       ),
   

4. Modify NewServeMux to accept a Route and use its provided pattern.

   // NewServeMux builds a ServeMux that will route requests
   // to the given Route.
   func NewServeMux(route Route) *http.ServeMux {
     mux := http.NewServeMux()
     mux.Handle(route.Pattern(), route)
     return mux
   }
   

5. Run the service.

   {"level":"info","msg":"provided","constructor":"main.NewHTTPServer()","type":"*http.Server"}
   {"level":"info","msg":"provided","constructor":"main.NewServeMux()","type":"*http.ServeMux"}
   {"level":"info","msg":"provided","constructor":"fx.Annotate(main.NewEchoHandler(), fx.As([[main.Route]])","type":"main.Route"}
   {"level":"info","msg":"provided","constructor":"go.uber.org/zap.NewExample()","type":"*zap.Logger"}
   {"level":"info","msg":"provided","constructor":"go.uber.org/fx.New.func1()","type":"fx.Lifecycle"}
   {"level":"info","msg":"provided","constructor":"go.uber.org/fx.(*App).shutdowner-fm()","type":"fx.Shutdowner"}
   {"level":"info","msg":"provided","constructor":"go.uber.org/fx.(*App).dotGraph-fm()","type":"fx.DotGraph"}
   {"level":"info","msg":"initialized custom fxevent.Logger","function":"main.main.func1()"}
   {"level":"info","msg":"invoking","function":"main.main.func2()"}
   {"level":"info","msg":"OnStart hook executing","callee":"main.NewHTTPServer.func1()","caller":"main.NewHTTPServer"}
   {"level":"info","msg":"Starting HTTP server","addr":":8080"}
   {"level":"info","msg":"OnStart hook executed","callee":"main.NewHTTPServer.func1()","caller":"main.NewHTTPServer","runtime":"10.125µs"}
   {"level":"info","msg":"started"}
   

6. Send a request to it.

   $ curl -X POST -d 'hello' http://localhost:8080/echo
   hello
   

What did we just do?

We introduced an interface to decouple the implementation from the consumer. We then annotated a previously provided constructor with fx.Annotate and fx.As to cast its result to that interface. This way, NewEchoHandler was able to continue returning an *EchoHandler.